The proposed study is designed to elucidate the interrelationships between cardiac lymph and acute myocardial infarction. To this end, we plan 1) to investigate various physiologic, pharmacologic, and pathologic mechanisms that may influence the volume and composition of cardiac lymph and 2) to determine the effects of such alterations in lymph on the subsequent changes that occur in the histologic, biochemical, and electrical characteristics of the ischemic heart. In experiments carried out during the current year we have determined the effects of methylprednisolone, 50 mg/kg intravenously, on the volume and composition of cardiac lymph in normal dogs subjected to acute myocardial ischemia produced by ligation of the circumflex coronary artery. Our findings do not support the hypothesis that high doses of steroids decrease the amount of lysosomes released into the interstitial space from ischemic myocardial cells. High doses of steroids, however, do appear to decrease the production of lactate and suppress the development of intramyocardial acidosis. Preliminary studies related to the effects of infusion of hypertonic mannitol indicate that this intervention increases substantially the volume of flow of cardiac lymph.